The precat manifolds are fairly restrictive. However it's the log design and not really the catalyst material. I missed those.
I would put them near the bottom of the "stock" list (above the heads) and right above the "SVT" parts on the "SVT" list.
I have cut my rear Y-split apart. It was well made and would pose no significant flow restriction as the 2.25" O.D. pipe splits to twin 2" O.D. piping. I would not even list it.
Other flow restrictions:
Stock airbox (it's so obvious though!)
Stock accordian tube
SVT accordian tube
SVT airbox
Stock MAF
